This invention relates to metal arch wires used for orthodontic correction of teeth wherein the wires are coated to have a color blending aesthetically with that of teeth.
Orthodontic treatment of improperly positioned teeth involves the application of mechanical forces to urge the teeth into correct alignment. The most common form of treatment uses orthodontic brackets which are small slotted bodies configured for direct cemented attachment to the front (labial) or rear (lingual) surfaces of the teeth, or alternatively for attachment to metal bands which are in turn cemented or otherwise secured around the teeth.
A resilient curved arch wire is then seated in the bracket slots. The restoring force exerted by the bent and twisted resilient wire tends to shift the teeth into orthodontically correct alignment. Depending on the shape of the arch wire (both round and rectangular cross sections are in common use) and the orientation of the bracket slot, it is possible to apply forces which will shift, rotate or tip the teeth in any desired direction.
Stainless steel is in many ways an ideal material for orthodontic brackets and wires because this metal is strong, nonabsorbent, weldable, and relatively easy to form and machine. Arch wires are also made of nickel-titanium alloys with a "shape memory" highly desirable in initial stages of orthodontic correction. A significant drawback of metal appliances, however, relates to cosmetic appearance when the patient smiles. Adults and older children undergoing orthodontic treatment are often embarrassed by the "metallic smile" appearance of metal brackets and wires, and this problem has led to various improvements in recent years.
One relates to development of adhesives, bracket bases, and techniques for direct cemented attachment of brackets to at least the anterior teeth which are prominently displayed when smiling. Direct cementation eliminates the need for metal tooth bands which are a major factor in the metallic-smile problem. Part of this has included development of smaller brackets which are less obtrusive.
Still another area of improvement involves use of non-metallic materials for the brackets. Ceramic orthodontic brackets have now been developed which are translucent and assume the color of underlying teeth so as to minimize appearance of metal in the mouth. The ceramic materials present a significantly improved appearance in the mouth, and often the only visible metal component is the arch wire. It is desirable to eliminate this last remaining metallic appearance from the anterior teeth.
It seems certain that metal wire will remain the material of choice for orthodontic arch wires because of its strength, stiffness and ductility. Thus, rather than a change of material, it appears that a coating to camouflage the metal is the only practical way of providing an aesthetic orthodontic arch wire. Wires have been coated with various non-metallic materials since at least as early as experiments with electromagnets and transformers. A variety of materials have been used for coating wires, but available coated wires are unsatisfactory for a commercially acceptable orthodontic arch wire. There are significant constraints on the properties of materials suitable for orthodontic use addressed in practice of this invention.
Plastic coating of orthodontic arch wires has previously been suggested. For example, U.S. Pat. No. 3,504,438 by Wittman et al. proposes replacing prior porcelain and acrylic coating materials for orthodontic devices with polytetrafluoroethylene (Teflon) or a material having similar properties such as fluorinated ethylenepropylene (Teflon FEP), trifluorochloroethylene, vinylidene fluoride, polyphenylene oxide, nylon, irradiated modified polyolefins and polycarbonates and other organic and inorganic polymeric materials (column 3, lines 32 to 38). It is stated to be desirable that the coating be colored similarly to the teeth and be characterized by the zero coefficient of friction of the Teflon. The Teflon is applied to a bracket, for example, by stretching and heat shrinking a Teflon sleeve. Spraying or dipping for the orthodontic devices is also contemplated. The thickness of the coating is unstated.
U.S. Pat. No. 4,050,156 by Chasanoff et al. describes an orthodontic arch wire coated with a layer of a mixture of para-oxybenzoyl homopolyester and polytetrafluoroethylene. This plastic may use titanium dioxide as a pigment. The thickness of the coating is not stated. The properties of the coating are said to be enhanced by the high content of thermoplastic fluorocarbon. In what may be a reference to the subject matter of the Wittman patent, the Chasanoff patent states that acrylics, porcelains and pure Teflon (polytetrafluoroethylene) coatings have been attempted, but have drawbacks such as lack of abrasion resistance, lack of stength, brittleness, and propensity to staining. It is not known that the product described in the Chasanoff patent has been commercially viable.
U.S. Pat. Nos. 4,585,414 and 4,659,310 by Kottemann describe an orthodontic arch wire in the form of an extruded plastic rod reinforced with a stainless steel wire core. It is stated that the stainless steel wire has a diameter in the range of from 0.008 inches to 0.014 inches, and after coextrusion with a plastic coating, the resulting product has an outside diameter in the range of from 0.016 inches to 0.022 inches. Thus, the thickness of the plastic is in the order of 0.004 inch (100 microns or micrometers). The '310 patent has an example where the wire core has a diameter of 0.011 inch and the total arch wire has a cross section of 0.018 inch yielding a plastic thickness of 0.0035 inch (89 microns).
The plastics employed comprise polysulfone and polyetherimide resins. It is conceded in the Kottemann patent that the resultant product is 27 times more flexible than stainless steel and 7 times more flexible than Nitinol. This, of course, means that much less corrective force can be applied to the patient's teeth due to the decreased properties of the arch wire. A product which is apparently similar to the product described in the patents is marketed by American Ortho under the trademark Filaflex.
U.S. Pat. No. 3,988,832 by Wallshein describes an orthodontic arch wire made in the form of a tightly wound helix of metal. It is stated that a soft coating may be placed on the exterior of the helix to protect the tissues in the mouth from the wire, as well as to prevent food particles from entering into the spaces in the wire. A plasticized and elastic material is used so as not to substantially affect the flexibility or working range of the arch wire. The thickness of the coating is not mentioned.
Prior soft, thick, thermoplastic coatings have not been satisfactory for appreciable use in orthodontics. As has been suggested, the required thickness of coating has reduced the stiffness of the wires and their suitability for orthodontic correction. A significant difficulty with prior attempts to coat orthodontic arch wires with plastics has been stripping of the plastic from the wire. During the course of orthodontic correction, the arch wire moves through the arch wire slot in the bracket attached to a tooth. This may occur during adjustment or correction. Such movement along the arch wire slot tends to strip the coating from the wire, much as one would strip wire for making an electrical connection. Small shreds of plastic coating are removed from the arch wire. Further, thick coatings tend to buckle on the inside of a bend in the arch wire, and that may be a location for failure of the coating.
It has also been proposed to apply an aesthetically appearing coating on just the "front" face of an arch wire. This enables the wire to have a full cross section for fitting in the arch wire slot of conventional brackets. A variation of this idea is described in U.S. Pat. No. 4,731,018 by Adell. The Adell patent describes a metal wire with a non-round cross section with a non-metallic part completing the circular cross section. Rectangular cross sections are also described.
Any such coating that extends around less than the full perimeter of the arch wire must have outstanding adhesion to the wire to prevent flaking or peeling. Further, with round arch wire, application of such a coating in a continuous process makes it difficult to assure that the coating is uniformly placed on the front face of the arch wire.
It is desirable to provide an aesthetically pleasing orthodontic arch wire overcoming such deficiencies in the prior art.